1. Field of the Invention
This invention relates generally to apparatus for completing downhole wells, and in particular to well screens for filtering unconsolidated material out of inflowing well fluid in water, oil gas and recovery wells.
2. Discussion of the Background
In the course of completing an oil and/or gas well, it is usual to first run a string of casing into the well bore and then run the production tubing inside the casing. At the site of the producing formation, the casing is perforated across one or more production zones to allow hydrocarbons to enter the casing bore. After the well is completed and placed in production, formation sand from unconsolidated formations may be swept into the flow path along with formation fluid. This sand is relatively fine and erodes production components in the flow path. In some completions, however, the well bore is uncased, and an open face is established across the oil or gas bearing zone. Such open bore hole arrangements are utilized, for example, in water wells, test wells and horizontal well completions. Similarly, after the well is completed and placed in production, formation sand from unconsolidated formations may also be swept into the flow path along with formation fluid.
With either cased or uncased well bores, one or more sand screens are installed in the flow path between the production tubing and the perforated casing. A packer may be set above and below the sand screen to seal off the annulus in the producing zone from non-producing formations.
Conventionally, the annulus around the screen in unconsolidated formations is packed with a relatively coarse sand or gravel which acts as a filter to reduce the amount of fine formation sand reaching the screen. To spot the gravel around the screen, a work string and service seal unit (SSU) is used. During completion, the service seal unit is employed to pump gravel through the work string where the gravel is squeezed into the producing formation around the screen. The gravel is pumped down the work string in a slurry of water or gel and is spotted to fill the annulus between the sand screen and the well casing. In well installations in which the screen is suspended in an uncased open bore, the gravel pack supports the surrounding unconsolidated formation.
However, with either cased well bores or uncased well bores, some spaces may remain that are not filled with gravel. These spaces eventually fill up with accumulated sand, which form sand plugs or bridges. Thus, fine formation sand finds its way through these bridges into the production flowpath and causes erosion, unless stopped by the sand screen.
In order to prevent fine formation sand from finding its way into the production, some sand screens employ a perforated mandrel which is surrounded by longitudinally extending spacer bars, rods or ribs and over which a continuous wire is wrapped in a carefully spaced spiral configuration to provide a predetermined axial gap between the wire turns. See for example, U.S. Pat. No. 3,785,409; U.S. Pat. No. 3,958,634; and U.S. Pat. No. 3,908,256. The predetermined aperture between turns permits formation fluids to flow through the screen, while the closely spaced wire turns exclude fine particulate material such as sand or gravel of a predetermined size which might penetrate the gravel pack surrounding the screen.
However, during the initial production period following the gravel packing operation, fine sand may be carried through the gravel pack before the gravel pack bridge stabilizes and yields clean production. Those fines tend to migrate through the gravel pack and screen and lodge within the inner annulus between the outer wire wrap and the perforated mandrel. In some instances, this can cause severe erosion of the screen and ultimate failure of the screen to reduce sand invasion. In other situations, the sand fines may include plugging materials which are carbonaceous, siliceous or organic solids which can completely plug the screen flow passages and terminate production shortly after completion. In deep wells, when the screen becomes plugged and the internal pressure in the production tubing is reduced, the formation pressure can collapse the screen and production tubing. Moreover, when a substantial amount of sand has been lost from the surrounding formation, the formation may collapse with resultant damage to the well casing, liner and/or screen, resulting in the reduction or termination of production.
One attempt to overcome the foregoing problem is to space the outer screen radially outward from the mandrel and interpose a prepack of gravel aggregate within the annular void formed between the inner mandrel and the outer wire screen. The prepacked gravel is sized appropriately to exclude the fines which accompany the formation fluid during initial production. Raw gravel, as well as epoxy resin coated gravel, have been used extensively in the annulus of prepacked well screens.
As disclosed in U.S. Pat. No. 4,917,183, it is also known to use spherical glass beads in prepacked well screens. However, it has been found that spherical glass beads are not well suited for gravel packing. The roundness of the glass beads creates problems when trying to hydraulically "stress" the gravel pack. In particular, if the outer surrounding screen has suffered erosion damage, thus exposing the annular prepack, the ball bearing effect of the glass beads, which acts as a natural lubricant, provides little resistance to channeling or movement of unconsolidated sands through the sand screen and into the production tubing, which ends in early erosive failure of the well screen and other production equipment. The glass beads are also susceptible to acid solubility, and most prepacked well screens are subject to retrieval problems due to their outer diameter being larger than that of a conventional well screen. In order to make prepacked well screens more easily retrievable, the inner mandrel is usually downsized, therefore creating restrictions in both production bore size and completion tool string bore size.
In some prepack arrangements, it is also known to utilize an inner wire cloth or steel woven fabric filter media in order to achieve maximum annular placement and retention of prepacked filter materials. See, for example, U.S. Pat. No. 4,858,691 and U.S. Pat. No. 4,856,591.
As disclosed in U.S. Pat. No. 5,004,049, a special clearance prepack well screen provides an outer wire wrap screen slightly larger than an inner wire wrap resistance welded screen. An advantage of the special clearance prepack is that the inner mandrel is only minimally downsized. The space in between, referred to as the "micro" annulus, is filled with an aggregate material. In such cases where this annular space is filled with loose (or non-consolidated) material, the aggregate is usually silica sand, glass beads, sintered bauxite, or nickel shot. However, if the outer screen suffers erosion damage, thus exposing the non-consolidated material, the prepack may provide little resistance to channeling or movement of unconsolidated sands through the sand screen and into the production tubing.
To avoid this disadvantage, these aggregate materials may be intermixed with a plasticized epoxy resin in order to consolidate the loose material, and prevent a bypass "channelling" effect when subjected to differential pressures, or when the outer screen suffers erosion damage. While epoxy bonding reduces the channeling effect, epoxy or formaldehyde based phenolic resin coated sands are acid reactive, often causing their bonding agents to disintegrate when exposed to acidic formation fluid or acidic workover fluids. Conventional prepack material such as silica sand is also acid soluble in hydrochloric acid (HCL), and is especially soluble in hydrofluoric acid (HF). Thus, acidizing procedures and other corrosive procedures which may be necessary to stimulate production, or other corrosive fluids which may be present in production fluids, may dissolve the prepack.